Acrylic rubber compositions have excellent balance among heat resistance, oil resistance, and price, and are widely used for sealing parts, such as O rings, gaskets, oil seals, and packings. Examples of acrylic rubber copolymers include those containing an epoxy group, an active chlorine group, an active chlorine group/a carboxyl group, or a carboxyl group as a crosslinking group. Among these, carboxyl group-containing acrylic rubber copolymers have a high vulcanization rate, and excellent heat resistance and excellent compression set characteristics. In addition, they are halogen-free acrylic rubbers that are non-corrosive to metals and are environment-friendly. Therefore, the demand for their use particularly as hose, sealing materials etc., is increasing recently. However, the scorch time (t5) is short relative to the vulcanization rate; that is, as the vulcanization rate increases, the scorch time tends to be too short, whereas as the vulcanization rate decreases, the scorch time tends to become long.
Generally, an aromatic diamine-based compound is used as a vulcanizing agent to secure sufficient scorch time. However, although the use of such a vulcanizing agent ensures scorch stability, the vulcanization reaction itself is slow, and there is thus a problem in productivity.
Patent Document 1 proposes a multivalent amine crosslinkable group-containing acrylic rubber composition using, as a vulcanizing agent, a diurethane compound that is a reaction product of hexamethylenediamine and 9-fluorenylmethyl chloroformate, and containing 1,8-diazabicyclo[5,4,0]undecene-7 or the like as a basic vulcanization accelerator. However, the vulcanization-molded product obtained from such a composition does not sufficiently satisfy the current compression set characteristics required for sealing parts.
Moreover, each example of Patent Document 1 discloses a molded product having improved compression set characteristics from an acrylic rubber composition, further using 1,3-di-o-tolylguanidine [DOTG] as a vulcanization accelerator. However, DOTG generates o-toluidine, which is a cancer-causing substance, by thermal decomposition. It is therefore concerned that DOTG is toxic to the human body, and the use of DOTG is already abolished in Europe. Accordingly, the use of DOTG may be globally restricted in the future.